1. Field of the Invention
The present invention relates to a manufacturing method of an organic-inorganic composite material and an optical material obtained by the manufacturing method.
2. Description of Related Art
In the field of organic-inorganic composite material, organic-inorganic composite materials wherein inorganic fine particles are dispersed in matrix of organic resin, so-called nano-composite materials, are receiving attentions since such materials can be expected to exhibit and improve various material properties such as mechanical strength, thermophysical property, gas permeability, optical property, etc.
As for a manufacturing method of such nano-composite materials, a composite method where inorganic fine particles are directly dispersed in a resin by Hot Melt Extrusion and a composite method (In-situ polymerization) where inorganic fine particles are dispersed in a polymerizable monomer for forming a resin and obtaining a resin through polymerization reaction under the existence of inorganic fine particles are suggested. The former is the major method for manufacturing polymer-clay nano-composite materials which are widely used industrially and the latter is suggested as a manufacturing method of, for example, silica/nylon nano composite materials, clay/polyolefin nano-composite materials, etc.
Further, there is also suggested a composite method performed in a liquid medium where resin is dispersed in a liquid medium, inorganic fine particles are further dispersed in the liquid medium and then the resin fine particles and the inorganic fine particles are flocculated (for example, see JP 2008-101049, JP 2007-119769, JP 2007-113010, JP 2005-232257 and JP 2006-089523).
On the other hand, in the field of optical material, as for an optical material having high permeability and high refractive index, change in linear expansion and refractive index due to temperature change is small, having excellent thermal stability and can replace glass, it is suggested to use a nano-composite material manufactured as described above. Such nano-composite material can improve refractive index and thermal stability as a raw material (for example, see JP 2009-084459).
With respect to a raw material of an optical material, it is required that a so-called nano-inorganic fine particles whose average particle diameter is 30 nm or smaller are evenly dispersed in a resin in order to obtain expected transparency and homogeneity as an optical material.
However, nano-inorganic fine particles are generally unstably dispersed and easily flocculated in resin, polymerizable monomer and liquid medium. Therefore, in a case where any of the above methods is used, it is extremely difficult to prevent flocculation of nano-inorganic fine particles when nano-inorganic fine particles are contained at high concentration.
Thus, in the above described conventional methods, it is difficult to prevent nano-inorganic particles from re-flocculating at the time of forming a composite by kneading and mixing and at the time of forming a composite by cast polymerization. When nano-inorganic fine particles are to be contained in a resin at a high concentration to obtain excellent refraction index and thermal stability, it is difficult to sufficiently disperse the nano-inorganic fine particles. Further, in the Hot Melt Extrusion, kneading and mixing need to be continued for a long time at a high temperature that allows sufficiently low resin viscosity in order to improve dispersibility of the nano-inorganic fine particles. However, under such kneading and mixing condition, it is difficult to prevent the resin from going through thermal decomposition, and the color of the obtained resin changes to yellow leading to a problem that an expected optical properties cannot be obtained.